You have a Marketo form embedded on a non-Marketo page. When you open developer tools in your browser you see:
"Error loading form: Object
message: "Subscriber <Your Munchkin ID> is not valid" "
The form embed code you are using is looking for your instance in the wrong place. Check the URL when you first log in to Marketo, make sure the sub domain in that URL matches what is in your embed code. For example: If you see https://app-sjst.marketo.com/ then make sure you have that same "app-sjst" in your embed code. Please note: This may vary between your sandbox and production instances.